1. Field of the Invention
The present invention relates to a method of manufacturing an X-ray mask and an X-ray mask blank, as well as to an X-ray mask and an X-ray mask blank manufactured thereby.
2. Related Art of the Invention
In the semiconductor industry, photolithography--whereby a minute pattern is transferred while visible light or ultraviolet light is used as electromagnetic radiation for exposure purposes--has conventionally been employed as a technique for transferring a minute pattern required for forming an integrated circuit on a silicon substrate.
However, in conjunction with improvements in technology, significant advances have recently been made in the field of semiconductor devices, such as ULSIs, thereby requiring a transfer technology which permits transfer of a high-precision minute pattern exceeding the limit of transfer of the conventional photolithography that uses visible light or ultraviolet light.
To attain transfer of such a minute pattern, an X-ray lithography which uses an X-ray shorter in wavelength than visible light or ultraviolet light has been developed and put into actual use.
The X-ray lithography is proximal exposure of a pattern to X-ray at X1 magnification, and hence an X-ray mask having X1 magnification is required. FIG. 1 shows the structure of an X-ray mask used for X-ray lithography.
As shown in FIG. 1, the X-ray mask 1 comprises an X-ray-transparent film 12 for permitting transmission of X-rays, and an X-ray-absorbing pattern 13a for absorbing X-rays. The X-ray mask 1 is supported by a support frame 11a formed from a silicon substrate. Further, in order to facilitate reinforcement and handling of the X-ray mask 1, a glass frame 15 whose outer diameter is greater than that of the support frame 11a is cemented to the support frame 11a. For instance, a support frame having an outer diameter of 4 inches and a glass frame having an outer diameter of 5 inches are used.
FIG. 2 shows the structure of an X-ray mask blank used for manufacturing the aforementioned X-ray mask. The X-ray mask blank 2 comprises an X-ray-transparent film 12, an X-ray-absorbing film 13, and an etching mask layer 14, all of which are laid on the silicon substrate 11 in this sequence.
So-called X-ray mask blanks also comprise a mask blank, such as that shown in FIG. 3, in which silicon located at the center of a pattern area in the X-ray mask is removed from behind and the support frame 11a is attached to the reverse side of the mask blank such that the X-ray film 12 is self supporting; and a mask blank, such as that shown in FIG. 4, including the glass frame 15 bonded to the reverse side of the support frame 11a beforehand.
As shown in FIG. 5, the X-ray mask 1 and the resist-coated wafer 3 are fitted to a longitudinal X-ray stepper such that the X-ray mask 1 and the wafer 3 are closely spaced about 20 .mu.m away from each other. After the X-ray mask 1 and the wafer 3 have been brought into alignment through use of alignment marks formed on both the X-ray mask 1 and the wafer 3, X-rays (synchrotron radiation in many cases) are radiated onto the wafer 3 byway of the X-ray mask 1, thereby sensitizing the resist that covers the wafer 3 and transferring a minute pattern onto the wafer 3.
In order to improve overlay accuracy between the X-ray mask 1 and the wafer 3 [including a tilt angle (i.e., an angle of rotation), in addition to alignment accuracy], accurate positional control of the X-ray mask 1 and individual corresponding stages the wafer 3 is required when they are secured on respective stages. To this end, as shown in FIG. 5, the X-ray mask 1, which is considerably heavier than the wafer 3, is correctly secured without distortion by chucking the glass frame 15 through use of a U-shaped handling arm 16. At the time of use of the handling arm 16, the portion of the handling arm 16 facing the X-ray mask 1 must be prevented from coming into contact with the surface of the wafer 3 during exposure. As a result of use of the X-ray mask 1 having the glass frame 15 shown in FIG. 5, the handling arm 16 chucks the glass frame 15, thereby preventing contact between the handling arm 16 and the surface of the wafer 3. The handling arm 16 facilitates transportation and holding of the X-ray mask 1 within the X-ray stepper.
The introduction phase of X-ray lithography has been delayed with recent progress in photolithography technology. At present, X-ray lithography is expected to be introduced into production of an X-ray mask used for manufacturing 1 GB-generation DRAM (having a line pitch of 0.18 .mu.m as specified by the design rule). X-ray lithography is characterized in that, even when introduced into production of an X-ray mask used for manufacturing the 1 GB-generation DRAM, it can also be applied to production of an X-ray mask used for manufacturing 4 GB-generation DRAM, 16 GB-generation DRAM, and 64 GB-generation DRAM. Assuming that X-ray lithography is applied to preparation of an X-ray mask used for manufacturing 64 Gbit DRAM, required positional accuracy becomes more strict, and positional accuracy as great as 10 nm would be required. Consequently, there arises a requirement to minimize to substantially zero the distortion in the X-ray mask stemming from the processes for manufacturing the mask. In order to establish a system to mass-produce semiconductors by use of X-ray lithography, an X-ray mask must be periodically cleansed for the purpose of eliminating contaminants which become attached to the X-ray mask while the X-ray mask is in use. The X-ray mask subjected to proximal X-ray exposure is required to possess a high degree of flatness. For instance, the inside of the silicon substrate requires a flatness of 3 .mu.m or less. Thus, there must be realized stable manufacture of X-ray masks having high positional accuracy and a high degree of flatness and allowing easy cleansing of the masks. In connection with cleansing of the X-ray mask, use of concentrated hot sulfuric acid or sulfuric acid-hydrogen preoxide mixture (a mixture of sulfuric acid and hydrogen preoxide) is said to be an effective cleansing method for removing contaminants such as organic substances.
An X-ray mask equipped with a reinforcement frame is usually manufactured by cementing the reinforcement frame to a support frame (i.e., a silicon substrate) of the X-ray mask through use of an adhesive such as epoxy. The adhesive is eluted during cleansing, thus contaminating the X-ray mask. Since cementing involves shrinkage of the adhesive, it is difficult to cement the reinforcement frame to the support frame while involving no warpage in the silicon substrate and ensuring good reproducibility. Consequently, a difficulty is encountered in stablely manufacturing highly-accurate X-ray masks.
There may also be conceived another method, in which an X-ray mask is manufactured from only a silicon substrate without use of a frame. However, since the surface of the silicon substrate is flat, such an X-ray mask poses a problem when being transported and held by a handling arm within the X-ray stepper or when being handled outside the X-ray stepper.